1. Field of the Invention
The invention relates to liquid crystal displays, and in particular to the method of eliminating disclination of liquid crystal molecules.
2. Description of the Related Art
FIG. 1 is a cross section diagram of a Bias-Bending Vertical Alignment (BBVA) type manufactured Liquid Crystal Display (LCD) panel. There is only one pixel 10 shown in FIG. 1. The LCD panel comprises a color filter 11, a liquid crystal layer 12, and an active matrix substrate 13. The color filter 11 and the active substrate 13 have a transparent substrate 111 and 131, respectively. The pixel 10 comprises a common electrode 112, a pixel electrode 134, and a control electrode 133. An insulation layer 132 is interposed between the pixel electrode 134 and the control electrode 133. VP, VCE and Vcom symbolize the potential of the pixel electrode 134, the control electrode 133 and the common electrode 112, respectively. The polarity of the pixel 10 is positive when VP is greater than Vcom. The polarity of the pixel 10 is negative when VP is less than Vcom. The difference between VP and Vcom, |VP-Vcom|, is determined by the gray level of the pixel 10. Vcom is constant. In pixel 10, the rotation of the liquid crystal molecules 121 are controlled by an electric field generated by the control electrode 133, the pixel electrode 134, and the common electrode 112. To prevent the liquid crystal molecules 121 from rotating reversely, VCE must satisfy the following requirements. First, if the polarity of the pixel 10 is positive, then VCE is greater than VP. Second, if the polarity of the pixel 10 is negative, then VCE is less than VP. Third and last, the voltage difference required between the control and the pixel electrodes (133 and 134) is dependent on the gray level of the pixel 10. The greater the gray level, the greater the required ΔV, wherein ΔV≡|VCE−VP|.
If any of the above requirements is not satisfied, the liquid crystal molecules 121 rotate in reverse as shown in area B of FIG. 1, hence, light transmission is reduced. The condition of reversely rotated liquid crystal molecules is referred to as disclination. Disclination results in a lower transmission ratio, a longer response time, and instability in the liquid crystal layer 12.
Published U.S. application US 2005/0083279 A1 disclosed various LCD panels based on the LCD panel of FIG. 1. The embodiments of US 2005/0083279 A1 satisfy the previously described first and second requirements but not the third requirement. The voltage difference between a control electrode and a pixel electrode is determined by several adjacent pixels of the pixel. The gray level of the pixel itself is irrelevant.
FIG. 2 shows several pixels of an embodiment of an LCD panel of US 2005/0083279 A1. The voltage difference between the control and the pixel electrodes of pixel 202 is determined by the gray level of the two adjacent pixels 204 and 206. ΔV≡|VCE−VP|=|V1−V2|, where V1 is the potential of a pixel electrode of the adjacent pixel 204, and V2 is the potential of a pixel electrode of the adjacent pixel 206. As shown in FIG. 2, both adjacent pixels 204 and 206 have a low gray level while the pixel 202 has a high gray level, wherein the pixel 202 suddenly changes from low to high gray level. Because the gray level of the adjacent pixels 204 and 206 are low, both V1 and V2 are close to Vcom, and |V1−V2| is very small. The pixel 202 requires a large ΔV to satisfy the previously described third requirement and thus prevent disclination. The actual ΔV, which is approximately equivalent to |V1−V2|, is too small to immediately rotate the liquid crystal molecules in the pixel 202. Disclination occurs in area 208. The backlight cannot completely transmit through the liquid crystal layer in area 208. When comparing area 208 with area 210, the pattern shown in area 208 is less sharp than the pattern shown in area 210, wherein area 210 shows an properly controlled liquid crystal molecule pattern and area 208 shows an improperly controlled liquid crystal molecule pattern. The disclination in area 208 results in image persistence.